Yersinia pestis, is the etiologic agent of plague, a disease that has killed millions of people during massive pandemics and persists worldwide. The pathogenicity of Y. pestis is largely dependent upon its ability to thwart the defenses of its host and to overwhelm it with massive growth. Two surface-exposed outer membrane proteins, Adhesion invasion locus (Ail) and Plasminogen activator (Pla) play a critical role in this process. This project focuses on Ail and its role in promoting Y. pestis pathogenesis. Ail is required for complement resistance, efficient attachment to host cells, Yop injection and for virulence in rodent models of bubonic, septicemic and pneumonic plague. Ail binds several host ligands including fibronectin, laminin and C4b- binding protein. We have recently identified vitronectin (Vn) as a new key ligand of Ail. Vn is a multifunctional protein with roles in cell adhesion and complement control. Vn also binds and stabilizes Plasminogen activator inhibitor 1 (PAI-1), a potent regulator of coagulation and fibrinolysis. Ail recruits Vn to the bacterial surface and promotes its degradation by Pla. The studies outlined in this proposal will: (i) use ligand-based NMR binding studies to define the Ail- Vn interaction at the molecular level; (ii) use bacterial cell-based and biochemical assays to define the role of the Ail-Vn interaction in fibrinolysis, complement resistance, cell attachment and Yop injection; and (iii) define the role of the Ail-Vn interaction in a murine model of septicemic plague. Ail is currently a target for therapy development and understanding how it functions is critical to this effort.